1. Field of the Invention
The present invention relates to a vehicular traction control system and in particular to a vehicular traction control system for a vehicle having first and second drive wheels driven through a common, open differential and first and second independently controllable foundation brakes for selectively independently retarding the rotational velocity of the first and second drive wheels, respectively.
2. Description of the Prior Art
The use of vehicular drive axles wherein a first and second (i.e. right and left) driven wheel are driven through an open differential is, of course, well known in the prior art. Examples of such drive axles may be seen by reference to U.S. Pat. Nos. 4,050,534; 4,018,097; and 4,651,587, the disclosures of which are incorporated herein by reference.
While such drive axles are very successful commercially and widely used in view of their economy, simplicity, desirability and ability to minimize wear on tires and the like, they are not totally satisfactory if one of the wheels is on a relatively low friction surface such as ice, loose gravel or the like. In such situations, as is well known, the wheel/tire on the low friction surface will spin, the open differential will divide drive torque equally and, thus the other wheel/tire will be provided with only the relatively low amount of torque developed by the spinning wheel.
To minimize the loss of driving torque experienced by an open differential drive axle in such "split friction surface" situations, various full or part time biased differentials of the "limited slip" or "locking" type are used. Examples of these types of devices may be seen by reference to U.S. Pat. Nos. 4,598,609; 4,569,250; 4,263,824 and 3,916,728, the disclosures of which are hereby incorporated herein by reference. While these modified differentials are effective to minimize loss of driving torque in split friction surface situations, they are not totally satisfactory as they are relatively complicated and/or expensive to provide, install and/or maintain; they tend to wear more rapidly than an open differential; they tend to cause drive shafts, axle shafts and tires to be exposed to increased wear and/or they require complicated and/or expensive controls for the automatic and/or manual operation thereof.
More recently, traction control systems have been proposed, usually in connection with vehicles equipped with an antilock brake system ("ABS"), wherein, if a wheel spin-out is sensed, the foundation brakes for that wheel are applied to stop the wheel and to allow an additional amount of torque generally equal to the braking torque to be supplied by the open differential to the other wheel. In certain of these systems, the torque output of the vehicle engine is also monitored and controlled. An example of a prior art vehicular traction control system may be seen by reference to U.S. Pat. No. 4,946,015, the disclosure of which is hereby incorporated herein by reference.
The prior art traction control systems were not totally satisfactory as the required sensors, logic units and actuators for monitoring and controlling engine torque may be complicated and/or expensive to provide and maintain and locking of the spinning wheel, often with an unnecessarily large braking torque, often resulted in causing the other wheel to spin as, especially in a heavy duty vehicle, the vehicular engine can provide sufficient torque to spin a single wheel on most, if not all, friction surfaces. Locking of the previously spinning wheel causes a significant vehicular drag and also wear on the wheel and brakes. Such a drag will increase the tendency of the other wheel to break loose and spin out. Further, it will result in unpleasantly rapid cycling and the requirement to dump and then rebuild air pressure as unlimited amounts of pressurized air are not available. Loss of pressurized air limits the traction control system capacity and may adversely effect the vehicle braking system. Also, it is likely that cycling will excite a natural frequency of the driveline and cause large stresses which may damage driveline components.